Recent research points out the potential importance of hormonal factors in the development of cognitive and affective disorders of aging. Animal models of the cognitive disorders of aging exist, and consequently it is desirable to study hormonal contributions to of the development of these deficits in animals. With traditional methods, it is extremely difficult to make meaningful measurements of the enduring endocrine status of an individual animal subject. These difficulties arise because the hormones to be measured are extremely reactive to stress, are normally secreted in an episodic or pulsatile way, and are predominantly present in the circulation in protein-bound forms that are not directly available to tissue. The proposed research aims to develop a simple, convenient, and reliable method for making continuous and repeated measurements over several days of the tissue availability of corticosteroid hormones in animal subjects. The proposed method uses an injectable, chronically implanted device to continuously accumulate the hormone of interest at a rate proportional to its interstitial fluid concentration. Accumulation of the hormone over some discrete time interval therefore reflects the time integral of its tissue availability. Since contact with the animal subject is gentle and only necessary at the beginning and end of a measurement interval, stress artifacts are minimized. Hardware for practicing this method can be offered commercially for diverse research applications, some of which may lead to eventual clinical applications.